Bike Medic

Figure out what's wrong with your bike — and what to do next.

Something wrong with your bike? Describe what you notice—or choose the part that's giving you trouble. Bike Medic starts with safe checks and helps you narrow down likely causes and work through what to try next, including when not to ride and when it's time for a bike shop.

See what Bike Medic gives you

Real output from an actual run on the problem below — an excerpt of a longer result, nothing reworded.

The problem

My rear derailleur is skipping under load when I push hard on the pedals, especially going uphill. Started after a wet ride last week.

Real run, 2026-10-08 — one of the tool's own built-in examples. Rated moderate, safe to ride.

What is probably going on

After a wet ride, water and grit work into the cable housing and increase friction, which changes the effective cable tension when you load the drivetrain. The derailleur then cannot hold the chain precisely on the sprocket under pedaling force, causing it to skip toward an adjacent sprocket and back. This is a cable-and-housing problem until proven otherwise — it is the most common mechanical consequence of riding in wet conditions and the cheapest to address.

Check these

  • Cable friction or contamination from the wet ride: check by shifting through all gears while stationary — if indexing feels sluggish or imprecise, or if the barrel adjuster is already wound well out from its original position, this is the likely culprit. Confirm by inspecting the housing ends for grit and by feeling whether the cable moves freely when you pull it by hand at the derailleur.
  • Chain wear: wet conditions accelerate chain wear, and a worn chain skips under load even with perfect indexing. Measure with a chain-wear indicator tool — replace at 0.5% stretch for 11 or 12-speed chains, 0.75% for 10-speed and below. If the chain measures worn, check the cassette too: a worn cassette will skip with a new chain until the cassette is replaced as well.

Fix it

  • SAFETY FIRST: Confirm the derailleur hanger is not bent before any adjustment. Look directly behind the bike at the hanger — it should be parallel to the cassette and frame. A bent hanger will not index correctly no matter how much you adjust, and riding with a severely bent hanger risks the derailleur being pulled into the wheel. If it is visibly bent, stop here and take it to a shop for straightening before proceeding.
  • Check the barrel adjuster on the derailleur (and the one at the shifter if present). Wind it all the way in (clockwise) gently, then back it out two full turns. This gives you a neutral starting point.
  • Shift through all gears in a stand or slowly on flat ground. If the chain hesitates to shift to larger sprockets (easier gears), turn the barrel adjuster counter-clockwise (out) a half-turn, then test again. Repeat until shifts are clean in both directions.

Pro tip

When you re-lube the cable, put the lube at the housing entry points and then cycle the shifter several times to work it in — lubing only the exposed cable section between housing segments does little for the friction that actually causes indexing shift.

When to take it to a shop

Take it to a shop if the derailleur hanger appears bent, if new cable and housing does not resolve the skipping under load, if the cassette shows worn teeth and you are unsure how to match a replacement to your drivetrain specification, or if you are not comfortable measuring chain wear accurately.

In a Nutshell

When
Something's wrong with the bike and the shop is closed.
You give
The symptom, or answers to a few diagnostic questions.
You get
The likely cause and a step-by-step fix with visual demos.

Good to Know

Questions people ask

Why does my bike chain skip under load?

The usual causes are a shifting cable with grit or friction in its housing (common after a wet ride), indexing that needs a small barrel-adjuster tweak, a bent derailleur hanger, or a worn chain or cassette. Check the hanger first, then the cable, then measure the chain.

When should I replace my bike chain?

Measure it with a chain-wear checker. A common rule is to replace at 0.5% stretch for 11- and 12-speed chains and 0.75% for 10-speed and below. A worn chain left too long wears the cassette too, and then both need replacing.

Which way do I turn the barrel adjuster?

If the chain is slow to shift to bigger sprockets, turn the barrel adjuster counter-clockwise (out) a half-turn at a time to add cable tension. If it is slow to shift to smaller ones, turn it clockwise. Test after each half-turn.

Is it safe to ride with a skipping chain?

Usually yes for a short, gentle ride, but a skip under hard pedaling can throw you off balance, and a badly bent hanger can pull the derailleur into the wheel. Bike Medic tells you whether it's safe to ride and when it needs a shop.